idea vs melody

idea and melody both are nouns.

Word	Noun	Adjective	Verb	Adverb
idea	Yes	No	No	No
melody	Yes	No	No	No

As nouns, melody is a hypernym of idea; that is, melody is a word with a broader meaning than idea:

Other hypernyms of idea include air, line, melodic line, melodic phrase, strain, tune.

idea (noun)	melody (noun)
the content of cognition; the main thing you are thinking about	the perception of pleasant arrangements of musical notes
your intention; what you intend to do	a succession of notes forming a distinctive sequence
an approximate calculation of quantity or degree or worth
a personal view
(music) melodic subject of a musical composition